It has now been 19 hours.

[Yahallo]

I don't see the issue with PoTD oO
(I don't know anything about computers so maybe that's why ^^")

Palace of the Dead uses resources on the server for its instances, the same as the solo instances. Shutting it down would allow for those resources to be repurposed for the solo instances, increasing the number of solo instances and number of people that would be able to progress through the main story. Palace of the Dead is likely the most popular instance due to how fast it allows DPS players, whom make up the majority of the player base, to level.

A way to see it is to imagine the instance server being a table, with each instance taking up space on the table. Unless there is enough space, an instance can't be placed on the table. By shutting down palace of the dead, you make more space for the solo instances.